1. Field of Invention
The present disclosure of invention relates to a method of forming an optical polarizing grid and to a liquid crystal display having the same, and particularly to a method of integrally forming a metallic polarizing grid by means of applying a nano-scale imprint lithography process to a substrate that has a relatively large surface area such as in the case of liquid crystal display panels.
2. Description of Related Art
A liquid crystal display (LCD) is typically composed of a thin film transistors (TFT) supporting substrate having pixel electrodes formed thereon, and a color filters supporting substrate having a common electrode formed thereon. The typical LCD also has a liquid crystal material layer inserted in between the TFT-supporting and filters-supporting substrates. A liquid crystal display can display images by applying appropriate voltages between the pixel electrodes and the common electrode at respective pixel areas so as to thereby rearrange the orientations liquid crystal molecules in between and thus adjust the amount of light transmitted at the pixel area and through the three major layers of the LCD (the TFT-supporting, the liquid crystal, and the filters-supporting layers). In backlit types of liquid crystal displays a backlighting unit is typically provided to the rear of the three major layers of the liquid crystal display for providing a source of polarized light to irradiate through the three layers.
Light irradiated from the backlighting unit is typically given specific polarization characteristics by passing the light through a polarizer before it enters the LCD's three major layers. Liquid crystal displays are able to display images by using a voltage modulated optical anisotropy of the liquid crystal molecules in combination with the light polarizing effects provided by the polarizer (or more correctly that provided by two polarizers; one at the bottom of the three layers and one differently oriented at the top).
Metallic wire grid polarizers have been developed in recent years for use as polarizers in liquid crystal display panels. Wire grid polarizers may be formed by forming on a substrate or a thin film, a striped pattern of parallel lines of metal or another reflective substance with line widths and inter-line spacings which are smaller than the wavelengths of red, green, and blue portions in the visible light that is perceptible by human. When unpolarized white light enters such a wire grid pattern from a backlighting unit for example, polarization occurs because such light generally travels with its wave oscillations extending perpendicular to the traveling direction. Only the light entering with its oscillations substantially parallel with the longitudinal direction of the spaces between wire grid polarizing patterns manages to initially pass through the grid.
When a wire grid polarizer is formed of a metallic material such as aluminum (Al) that has a high optical reflectance, light entering from the backlighting unit and with its oscillations roughly perpendicular to the longitudinal direction of spaces between wire grid polarizing patterns cannot pass through the spaces and instead reflects back to the backlighting unit. If a phase-altering transmission layer (modulation layer) is provided having different refractivity and being disposed between the wire grid polarizer and the backlighting source, then the phase of the back reflected light is changed as it passes into the phase-altering transmission layer and part of that light refracts back up to try and re-enter the wire grid polarizer at a new polarization angle. After one or more tries it succeeds and thus an additional passage of polarized light occurs as a result of such phase-changed reflection or refraction.
Recycling of light as described above may be continuously carried out, so that the wire grid polarizer has a similar effect as DBEFs (Dual Brightness Enhancement Film) that improves the transmittance of polarized light. Accordingly, because recycling of light can be embodied using a simple modulation layer structure instead of using the prior-art, but complicated DBEFs, an inexpensive polarizer having high transmittance can be achieved.
However, because in this application a metallic reflective film typically needs to be formed by patterning a metallic layer at a nano scale of about 50 nm to 200 nm, such a wire grid polarizer cannot be reliably manufactured if the metal patterning process does not provide sufficient fine resolution and repeated ability during mass production. The called for, small dimensions suggest that one should provide a photosensitive layer atop a reflective layer, that one should photo-lithographically pattern the entire layer at the nano scale and that one should etch the metallic reflective film by using the patterned photosensitive layer as an etch mask.
Recently, a nano imprint lithography process has been made available that patterns a photosensitive film for subsequent development by press-bonding a small stamp having a desired pattern engraved in a bonded-part thereof and by press-applying the stamp to the photosensitive film. In such a nano imprint lithography process, mechanical accuracy is very important because a pattern is formed by mechanically press-bonding the stamp to the photosensitive film. As for small parts, such as or wafers for semiconductor integrated circuits (i.e., i.e. of diameter less than 12 inches), mechanical accuracy across long distances (greater than about 300 mm) is not very important. However, in the case of Liquid Crystal Displays (LCDs), process yield appears to depend considerably on long-distance mechanical accuracy because the substrates have relatively large surface areas (i.e., much greater than 600 mm×720 mm) which correspond to the whole of the liquid crystal display panel that is viewed by users. Therefore, in order to form a nano-scaled photosensitive pattern on a substrate having such a large area, it appears that a very accurate process needs to be developed for dealing with dependence on mechanical accuracy over long distances. In other words, although the existing nano imprint lithography processes might be effectively used for small-sized substrates such as conventional IC chips or conventional wafers, it does appear that the same processes can be applied for forming wire grid polarizers across substrates having a much larger area, because the mass production yield will decrease significantly and uniformity in line widths and inter-line spaces of the wire grid polarizing pattern will likely decrease if one attempts to roll a stamp across such a large surface area. In addition, it appears that polarizing characteristics of the polarizer will also be decreased by lack of uniformity in a wire grid polarizing pattern produced by such conventional application of the nano-imprint technique.